Industrial locomotives are usually much smaller than road locomotives widely used on Class I or shortline railroads. Such industrial locomotives are used for switching or shunting cars at industrial sites. They are also widely used to haul bulk materials in quarries, mines or even tunnels under construction. The track for such locomotives may be narrow gauge, and in many instances temporary. All too frequently the track is substandard and has considerably less than an ideal geometry. Moreover, the track is often built in less than ideal space and environmental conditions. Derailments are frequent and rerailments difficult. Some of the track may even be made of wooden rails.
The suspension of the locomotive wheel sets for very rough trackage creates special problems, particularly if the required wheel adhesion or tractive or braking effort is to be maintained. The problem is particularly acute in a direct drive locomotive where the wheel set may include a housing, differential, and a drive shaft connected to the differential. It is important that both wheels of the set engage the rail with enough force to prevent wheel slip, and also be afforded a relatively wide range of canting movement or rocking in a vertical plane such as might be caused when one rail is lower than the other, or such rails are out of transverse gauge or alignment. In a direct drive locomotive, it is desirable that the canting of the wheel set be as nearly as possible about the drive shaft connection or the lateral center of the wheel set. This is particularly important in a direct drive locomotive where both the driving and braking forces are transmitted from the locomotive chassis.
Since the weight of half of the locomotive may be carried by a wheel set, the spring system between the wheel set and the frame should permit such extreme canting and be positioned in each side of the wheel set so that only the spring system on one side may support substantially the entire weight of one-half of the locomotive, with the other side literally lifting off its seat between the frame and wheel set. While this accommodation of such rather wide ranging movement of the wheel set for rough trackage is important, it is also important that the spring or snubber system taking a substantial portion of the weight of the locomotive permit this motion while transferring the weight of the locomotive to the rails to maintain wheel adhesion and tractive effort for both driving and braking forces. Accordingly, the spring snubber system, in addition to being properly located, should also have the capacity and range of movement to accommodate the conditions noted above.